Energy efficiency has been a huge issue in modern home design ever since the energy crisis of the 1970’s. This crisis was brought about by an oil embargo by major oil producing countries that were members of OPEC, the Organization of the Petroleum Exporting Countries.
This embargo raised energy prices significantly around the world and sparked a movement in the United States to reduce dependence on foreign oil by reducing energy use.
Improving the energy efficiency of houses played a major part in the effort to reduce energy use. Since the embargo there has also been a parallel movement to improve the Earth’s climate. A large part of this movement focuses on reducing the use of fossil fuels.
These efforts have looked at improvements in home insulation, reduction or elimination of unnecessary air infiltration, improved windows and doors, and the application of new materials to reduce the impact on energy use and the environment.
One item that’s not been discussed much and has definitely not been applied to most house designs is the shape of the house and how it relates to energy efficiency.
Looking to the past
Photo by Hu Totya, Wikimedia Commons:
www.creativecommons.org/licenses/by-sa/2.5/deed.en
An interesting thing about looking at the shape of the house in relation to energy efficiency has to do with how this was applied historically.
Most common houses in northern climates have traditionally been rectangular and compact in shape through the mid-twentieth century. Though much of this has to do with limitations of obtaining materials and what could be reasonably built at those times, the compact shapes also benefited energy use.
Early American homes were primarily heated by fireplaces or cast iron stoves until the twentieth century. The use of wood, peat, and then coal to burn for heat was based on availability.
Small compact houses minimized the use of fuel since these forms limited the amount of exterior surfaces in relation to the interior volume. Since insulation was limited to non-existent minimizing exterior surfaces was important. This compact aspect of houses continued through the 1940’s, which you’ll notice in older neighborhoods.
Today there are people who push for maximum energy efficiency or even zero-net-energy use, much of which can be improved with the shape of the house and the materials used to build it.
Basic concepts of energy efficiency
Energy efficiency boils down to the balance between the materials used to build the house and their performance in relation to the amount of energy used by the occupants of the house.
Performance of the materials has to do with:
- The shape and size of the house – the amount of material enclosing the house.
- The materials used to build the exterior walls of the house
- The amount of insulation provided in the exterior walls, the roof, and the foundation
- The amount of air leakage through the wall
- The amount of air infiltration allowed by exterior doors and windows
- The performance of glass in windows and doors
- The orientation of the house in relation to the sun and local climate forces
The goal is to provide the least amount of heat gain from the sun during summer, capturing an effective amount of radiant heat from the sun in winter, and keeping cold and heat from transmitting through the wall while allowing the interior to remain comfortable. Reducing or eliminating the amount of air leaking into the house and inside air leaking out is part of this.
Shape
The importance of the shape of the house has to do with the amount of exterior surface used to enclose the house. A house with a significant amount of exterior wall compared to interior volume has a bigger challenge to balance its performance. Let’s take a look at what this means.
The dome shape
Photo by Mr. Satterly, Wikimedia Commons: WTFPL license
The sphere has one of the least amounts of surface area compared to its volume; however, the shape is not easily constructed or utilized.
Domes, which are essentially partial spheres, can be built somewhat easily . A half-sphere is best for a house to have the greatest amount of usable space.
The most famous application of a dome form used for houses is the geodesic dome. This structure was created by Buckminster Fuller and is based on using triangular elements to create the shape.
The round shape
The round shape is the most efficient of the non-dome shapes. Its surface area for 10,000 cubic feet at 10 feet of height, including a flat roof, is 2,121 square feet.
To get an actual round shape you’d have to use masonry such as brick or stone or materials that can bend to form the curve of the shape such as metal panels. Otherwise, round shapes are typically constructed of straight segments to allow for more typical materials and construction methods.
The two-story square box
Photo by W. Marsh
The multistory square box with the overall height maximized in proportion to its horizontal length is the closest rectangular form to the sphere.
For this analysis we’ll use a two-story house with exterior walls of equal length and 20 feet high on all four sides (10 feet per level) and a flat roof. This two story box contains 10,000 cubic feet and has an exterior area (four walls plus roof) of 2,289 square feet.
The one-story square box
A one story box containing 10,000 cubic feet with exterior walls of equal length and 10 feet high on all four sides with a flat roof has an exterior area of 2,265 square feet, slightly less than the two-story square box shape.
Rectangular shapes
As we saw above, the difference between a one and two story house of the same shape and volume is small.
Lets now look at rectangular shapes to see how different sizes of the same volume can make a big difference.
Using the same 10,000 cubic feet of volume and 10 foot floor heights, lets start with a 1,000 square foot one story house. There are multiple lengths of walls that can be used to get the square footage.
A rectangle that’s 30 feet wide by 33.33 feet long gives us 126.67 feet of wall length to enclose 1,000 square feet. However, a rectangle that’s 20 feet wide by 50 feet long gives us 140 feet in length for the same number of square feet. The difference in length is 13.34 feet, which multiplied by its height of 10 feet is 133.4 square feet. This is quite a difference in surface area to obtain the same volume, equating to the floor area of a 10 x 13 room!
More complex shapes using the same criteria for height and volume increases the size of the exterior enclosure. With this knowledge, keeping shapes simple is better for energy efficiency.
Earth sheltered design
Wikimedia Commons: https://creativecommons.org/licenses/by-sa/4.0/
The earth sheltered house is one that’s built into a slope or hill, usually such that only one wall is exposed to the exterior. The balance of the house is built against soil for the walls with the roof covered with soil.
Dirt is not a good insulator; you can infer this from the fact that frost can form in soil many feet below the surface. However, it’s a good material to block strong winds that can reduce energy efficiency in a more typical house. Soil can also provide a more stable temperature at greater depths which can assist with modulating interior temperatures during hot and cold weather.
The soil covering the roof is not used for insulation due to minimal insulation properties and freezing soil, Rather it’s used to assist in protecting the house from wind and extreme radiant energy from the sun.
The impact of roofs
Photo by Cayl Hollis
Roofs can also impact the energy efficiency of houses. However, the form of the roof is less important than how well the house is insulated.
The standard type of roof for most homes is a gable roof or hip roof which have slopes on at least two lengths of the house. These roofs are effective at shedding water off the house and shedding snow with steeply-pitched roofs.
Photo by McHeath, from Wikimedia Commons
These roofs are typically ventilated, meaning there are openings that allow for air to flow through the attic space created by the roof and the ceiling joists below. Horizontal insulation for gable or hip roofs occurs in the horizontal ceiling joists that run beneath the sloped roof framing, typically with batt insulation fitted between the wood joists.
Another type of roof is the “flat roof”. This type of roof simplifies framing; however, horizontal insulation has to be handled differently. One option is to use batt insulation underneath the roof sheathing and placed between the roof joists. A method becoming more common is using insulation board (rigid insulation) atop the roof sheathing that’s placed over the roof joists.
Using insulation board above the roof sheathing allows for a more uniform insulation membrane at the roof. Typical roofs such as gables and hips, though they use insulation in the ceiling joists, do not have a uniform insulation membrane due to the joists between the insulation interrupting the uniformity of the insulation.
The flat roof can be tilted for better drainage, creating what is called a shed roof.
From photo by Marcoandradefilho, from Wikimedia Commons
Improving wall performance in traditional construction
Wood framing is the most common means of building houses in the U.S. Prior to World War II homes were built with rudimentary insulation, and decent insulation was provided after. However, these walls were rarely fully air tight.
The impact of the energy crisis in the 1970’s brought about a call to improve insulation in houses. To do this we increased the amount and performance of insulation. We also made the overall wall system, including doors and windows, more airtight.
Making walls more airtight saved energy, but created new issues with moisture that had to be resolved.
Framed construction is made of wood components for the structure and wood panels as sheathing to create the enclosure for the house. Houses prior to the energy crisis were less airtight than today, which allowed for any small amounts of moisture that got into the framed wall to dry out before damage occurred.
Making our houses more airtight created a problem with trapping moisture inside the exterior walls. This is due to continuous sheets of plastic now commonly used as air and moisture barriers over the exterior wall sheathing. Making the joints around doors and windows more airtight completed the system. However, this would trap any moisture that got into the wall rather than letting it dry out.
Most of the moisture getting into walls was generated inside the house. Cooking, bathing, and air humidification in the winter would create moisture in the form of a vapor which would get into the walls. With the outside sealed tight the moisture would linger in the wall causing damage to the wood and sometimes creating an environment for mold growth.
To resolve the water vapor issue a plastic vapor barrier sheet was developed. This is placed over the inside face of the wood framing members prior to installation of the gypsum board wall sheets, with carefully taped joints and gaps.
The impact of glass
In addition to improving how walls are constructed, windows have also been improved to reduce the amount of cold or hot air transmitting through them. This was done by using insulated glass which consists of two sheets of glass tied together by a perimeter frame that’s then placed into the window unit.
The gap between the glass creates a bit of “insulation”, with the air inside acting to reduce the transmittance of temperatures compared with windows using one sheet of glass.
The impact of materials
Though most exterior finish materials used on houses have not acted to improve energy efficiency to any great extent, a few new types of wall construction have been created to do that.
Insulated block walls use the structural masonry method of construction but provide integrated insulation in the blocks to reduce the amount of additional insulation needed for the exterior wall.
Structural insulated panels use a sandwich of rigid sheets of wood on either side of rigid insulation board to create structural sheets that can hold up floor and roof loads. This creates a more uniform and airtight insulating system than typical construction.
The impact of house orientation
Beyond designing an energy efficient form for your house, another important factor to maximize the efficiency is the orientation of the house on your property.
Day-to-day spaces such as living rooms can be oriented toward the sun in cooler climates or away from the sun in hot climates. Using the sun to warm rooms in colder months will reduce energy use, using shading devices to help with cooling in the summer.
Orientating your living spaces facing the sun but turned slightly to the east allows for morning warming during colder months and minimizes afternoon heat gain in hotter months.
The impact of building mass
The vast majority of houses in the United States are constructed with wood framing. A counter-option is to use building mass.
Building mass is the use of masonry, such as brick or solid block, or concrete to create thick perimeter walls of the house. The material of these walls is called mass.
Photo by Craig Klomparens, courtesy Tilton, Kelly + Bell
One benefit of mass has to do with temperature lag. Mass takes time for temperature to make its way through it and can retain temperature longer than framed construction.
For example, when temperatures get hot, the inward side of the mass will stay cooler for an extended period of time before it gets warm, thereby delaying the need for cooling.
The opposite holds true as well. When temperatures get cold, the inward side of the wall will stay warm for a longer period of time before heating is needed.
An additional benefit of mass is retaining heat to use later. A mass wall that heats up during the day can radiate that heat into the house during cooler nights.
Though mass walls have these benefits they’re not as effective as insulation at keeping indoor heat from eventually making its way through a wall. Many houses built with masonry or concrete walls still need to provide insulation as part of the wall system.
Masonry mass can still be used as an interior wall. Interior mass walls that can receive direct sunlight from clerestory windows can retain and radiate the solar heat back into the house at night. This benefit during the colder months can reduce heating loads for the house.
Balance
Though trying to maximize absolutely everything in your house for the most energy efficiency is noble. However, living in a small box is not the dream for most of us. Having a reasonable balance between design and energy efficiency is a better goal so that you can have a house you’ll enjoy living in.
All graphic images by Cayl Hollis.
Top image: Photo by Marcoandradefilho
Wikimedia Commons
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